Brazil's team doctor said a hotel mattress may have caused the back spasms that forced Marcelo off the field during the 2-0 victory over Serbia.
Bed troubles for Marcelo
The Real Madrid left-back was substituted after just 10 minutes of the game and early analysis suggested he was suffering from back problems.
Dr Rodrigo Lasmar told reporters after the game that he was optimistic over the 30-year-old's chances of being fit for Brazil's round of 16 clash with Mexico on Monday, but Marcelo's sleeping conditions might need to be assessed.
"It is still too early to make an accurate diagnosis," said Dr Lasmar. "The injury may be related to the hotel mattress.
"Although our expectation is good, but we have to be calm and wait for the next few days."
Marcelo missed three Real Madrid matches due to a hamstring injury in February but has been otherwise largely injury free in recent years.
"No recovery period" says doc
Dr Lasmar said that as long as a serious injury didn't emerge, he would treat Marcelo's pain in order to prepare him for the Mexico game.
"Marcelo is medicated and with medication the tendency is that the response is good and that he can play," he said. "The important thing is that there is no injury.
"They were muscle spasms. Once he feels good to train, he will return and immediately be ready to play.
"There is no recovery period."